INFLUENCE OF THE DIPOLE-DIPOLE INTERACTION ON VELOCITY-SELECTIVE COHERENT POPULATION TRAPPING

We analyze the influence of the dipole-dipole interaction between grou nd and excited state atoms on atomic cooling by velocity-selective coh erent population trapping. We consider two three-level atoms in the La mbda-configuration, interacting with two counterpropagating laser fiel ds as well as with the electromagnetic vacuum modes. The elimination o f these modes in the Born-Markov approximation results in spontaneous decay, which is essential in providing the momentum diffusion necessar y for cooling, as well as a two-body dipole-dipole interaction between ground-and excited-state atoms. The corresponding two-body master equ ation is solved numerically by Monte-Carlo wave-function simulations. Our main result is that although a dark state survives the inclusion o f dipole-dipole interactions, the presence of this interaction can sig nificantly slow down the cooling process for sufficiently high atomic densities.